Removing a heat sink from a cpu using a butterknife. (Only thing I had to use at the time....butterknife slipped off the retainer clip, and stabbed the motherboard=dead motherboard) Yeah, I know, I know, lol!

My most expensive mistake at work (i'm a techy) but not entirely down to computers... one of our photocopiers stopped working and throwing errors about waste toner. I popped it open and had a look, found something that looked like waste toner in 4 compartments so i proceeded to empty all 4 of them into a bin, i replaced the units and still wasn't fixed so we called an engineer.. He said 'someone' had emptied the fusing/magnetic something or other material out of each toner unit, which was a costly mistake. It was £250 per unit and 4 units needed replacing so it cost us £1000, i seriously thought i was going to lose my job but my manager was understanding. In my defence it's my responsibility when i don't know much about photocopiers at all.

Another costly mistake was not having up to date virus software on all of our machines, we got stund with a dodgy USB disk and it ripped through our network infecting every single PC, it didn't really cost us any money but it certainly took me a long time to fix.

Also when i was just curious as a child i was messing around with the insides of my brand new PC and when re-seating the graphics card i pushed too hard and cracked the motherboard.

I've been quite lucky really. I think the worst thing I ever did was accidentally install Grub to a Windows partition, but that wasn't too difficult to fix. Still I was cursing myself at the time for being so careless

I was getting jiggy with an ex-girlfriend. I put candles all over my desk and someone knocked the door. Quickly got dressed, hit the pause button and eject cd button at the same time by accident. Came back up stairs and smelled something odd. Turned out it had melted my cd drive. In a panic, I pushed it back in. By the time it had cooled, I imagine it had basically turned into a glue totally sealing it permanently.

It was an eMac so I was fairly screwed.

Oh also, the first real PC I had which was around 1998 and I was 12. I knew nothing of computers really. Messing around with all the buttons and everything, switched the voltage on the PSU and switched it on. There was a loud bang and a puff of green smoke. I'd been waiting for six months to get it. It was Christmas day and I'd had it for a few hours. I cried like a baby.

-Installed Windows 8 -Volt-modded a Radeon 9500 Pro that died after 6 months on a hot summer day (I still have the bookmark http://www.xtremesys...=9500Pro%20volt)-Thought Windows 8 would be an enhanced, better version of Windows 7-Chose a Voodoo 3 over a TNT2 when I bought my old P3 550MHz. The Voodoo wasn’t a bad card, but the TNT2 was superior-****** up the line-in jack of a brand new Xonar Essence STX with a bad connector-Made an ugly scratch on my screen with a… hard disk -Chose to buy 4 x 1GB of RAM instead of 2 x 2GB on my old system, to save money. A year and a ½ later, I had to buy 4 x 2GB anyway

Nothing overly dramatic, probably my personal least favorite just because it was my hardware.. had a "derp" moment couple years back, made an ugly short in the eletrical. Figures that I can build hundreds of the things for clients over the past couple decades, and wind up nuking my own.

Dual socket motherboard, probably around $400 or so at the time.
Dual CPU's, maybe $500 for the pair.
That look on my face when I first got a whiff of the blue smoke, priceless.